1. Field of the Invention
The present invention relates to a liquid crystal display, and particularly, to a liquid crystal display of a bend alignment mode.
2. Description of the Related Art
Liquid crystal displays (LCDs) have great advantages such as thinner profiles, lighter weights, and lower power consumption as compared with cathode ray tubes (CRTs). The liquid crystal display has a liquid crystal cell and a pair of polarizing plates disposed on both sides of the liquid crystal cell. The liquid crystal cell includes liquid crystalline molecules, two substrates for enclosing the liquid crystalline molecules, and an electrode layer for applying voltage to the liquid crystalline molecules.
Moreover, an alignment film is usually provided on each of the two substrates so as to align the enclosed liquid crystalline molecules.
Moreover, an optical film (phase retarder) is often provided between the liquid crystal cell and the polarizing plate so as to eliminate coloration of an image to be displayed on the liquid crystal cell.
A laminate of the polarizing plate (polarizer) and the optical film functions as an elliptically polarizing plate. Moreover, in some cases, the optical film is provided with a function to enlarge the viewing angle of the liquid crystal cell. As the optical film, a stretched birefringent film has been conventionally used.
It has also been proposed to use an optical film having a first optical anisotropic layer containing a discotic compound in place of the stretched birefringent film (see Japanese Patent Application Laid-Open (JP-A) No. 06-214116, U.S. Pat. Nos. 5,583,679 and 5,646,703, and German Patent Application No. DE 3 911 620).
The first optical anisotropic layer is formed by aligning a discotic compound and fixing its alignment state. This discotic compound generally has a large birefringence. Moreover, the discotic compound can be in various alignment modes.
Therefore, using the discotic compound makes it possible to manufacture an optical film having optical properties that cannot be obtained by the conventional stretched birefringent film.
For example, liquid crystal displays using liquid crystal cells of a bend alignment mode where rod-shaped liquid crystalline molecules are, in upper and lower parts of the liquid crystal cell, aligned in substantially opposite directions (symmetrically) have been proposed (see U.S. Pat. Nos. 4,583,825 and 5,410,422).
In such liquid crystal displays, since the rod-shaped liquid crystalline molecules in the upper and lower parts of the liquid crystal cell have been aligned symmetrically, the liquid crystal cell of the bend alignment mode has a self-optical compensatory function. Therefore, this liquid crystal mode is referred to also as an OCB (Optically Compensatory Bend) liquid crystal mode.
The liquid crystal displays of the bend alignment mode have an advantage of a high response speed.
The bend alignment mode has features in a wider viewing angle and a higher response speed as compared with the common liquid crystal modes (TN and STN modes).
However, in comparison with the CRTs, the liquid crystal displays of the bend alignment mode require further improvement. In order to further improve the liquid crystal display of the bend alignment mode, it can be considered to use an optical film as in the common liquid crystal modes.
However, the conventional optical film made of a stretched birefringent film was insufficient in the optical compensatory function for the liquid crystal display of the bend alignment mode.
As described above, it has been proposed to use an optical film having an optical anisotropic layer containing a discotic compound and a support in place of the stretched birefringent film.
Furthermore, liquid crystal displays of the bend alignment mode using optical films containing discotic compounds have also been proposed (see JP-A No. 09-197397 and International Publication No. WO 96/37804).
Using the optical films containing discotic compounds allows significantly improving the viewing angles of the liquid crystal displays of the bend alignment mode.
A problem has been pointed out that light with a specific wavelength leaks to cause coloration of a display image when an optical film containing a discotic compound is used for a liquid crystal display of the bend alignment mode (see JP-A No. 11-316378).
There is a description to the effect that the cause for this coloration exists in wavelength dependence of the transmittance of an elliptically polarizing plate (a laminate of a polarizer and an optical film).
And, it has been reported that the maximum optical compensatory effect on a liquid crystal cell of the bend alignment mode can be obtained by disposing the first optical anisotropic layer and the polarizer so that an angle between an average direction of orthographic projections of the normal lines to discotic planes of the discotic compound onto the first optical anisotropic layer and an in-plane transmission axis of the polarizer becomes substantially 45°.
Moreover, various methods have been proposed to reduce a change in hue and prevent tone reversal of a bend-alignment liquid crystal display using an optical film containing a discotic compound (see Japanese Patent (JP-B) No. 3056997, JP-A Nos. 2002-40429, and 2006-243179).
In recent years, applications of liquid crystal displays have been expanded, and liquid crystal displays have been adopted in, for example, car navigation systems, instrument panels, rear-seat gaming applications, and Head-Up Displays.
Therefore, in bend-alignment liquid crystal displays for such applications, it becomes necessary to realize a gradation display with a higher degree of freedom, so that a further improvement in transmittance is demanded.
For improving a liquid crystal display in transmittance, there is a method for increasing the liquid crystal cell in Δnd and a method for increasing the optical film in Rth (see JP-A Nos. 09-211444 and 2006-243179).
However, in the method for increasing the liquid crystal cell in Δnd, it is necessary to optimize optical characteristics of the optical film according to the value Δnd of the liquid crystal cell.
In addition, as the method for increasing the optical film in Rth, when an additive to develop such optical characteristics is added, manufacturability is lowered due to bleeding of the additive and a haze degradation, so that realization has been difficult.